The present invention relates to the general field of composite-material blades for a turbine engine rotor wheel. The invention relates more particularly to damping the vibration that appears in operation between the outer platforms of two adjacent blades of a rotor wheel.
A turbine engine rotor wheel, e.g. a rotor wheel of a low-pressure turbine stage of a turbojet, comprises a disk having a plurality of blades mounted thereon. At their free radial ends, each blade has a transverse element referred to as an outer platform, which element serves in particular to define the outside of the flow passage for the gas stream passing through the turbine.
The outer platform of such a blade has an upstream edge and a downstream edge extending perpendicularly to the flow direction of the gas stream. These edges are connected together via two side edges via which the outer platform of the blade comes into contact with the outer platforms of the two blades of the rotor wheel that are directly adjacent thereto.
In general, with metal blades, the side edges have a so-called “Z-profile”, i.e. each of them has two axial portions that are interconnected by a portion that is substantially transverse. In order to damp the vibration to which they are subjected while the turbine is in operation, it is known to mount the blades on the disk with a certain amount of twist about their main axes. At the outer platform of any one particular blade, this twist stress serves to put the transverse portions of the outer platform of the blade into contact with the transverse portions of the outer platforms of the adjacent blades. The contact and friction forces as generated in this way at the outer platforms of the blades serve to dissipate the vibratory energy resulting from operation of the turbine.
Such vibration damping is nevertheless not applicable to rotor wheels in which the blades are made of composite material. In particular for a ceramic matrix composite (CMC) blade the stresses generated by twisting the blade are excessive compared with the capacity of the composite material. Furthermore, having recourse to blades that are made of composite material also has the drawback of giving rise to large radial offsets or to large displacements between the outer platforms of adjacent blades in the event of mutual tilting between the blades.